1. Field of the Invention
The invention relates generally to the field of seismic energy sources. More particularly, the invention relates to impulsive seismic energy sources using electromagnets to move a mass.
2. Background Art
Seismic surveying includes imparting seismic energy from a seismic energy source into rock formations below the land surface, or below the bottom of a body of water in marine environments. The seismic energy travels from the source, through the rock formations and is reflected from acoustic impedance boundaries in the subsurface. Such acoustic impedance boundaries are typically at the interfaces of layers of different rock formations. The reflected seismic energy is detected by a plurality of seismic receivers disposed at the surface, on the water bottom or in the water. The detected seismic energy is interpreted to infer, among other things, structure and composition of the rock formations below the surface or water bottom.
Seismic energy sources known in the art include “impulsive” sources. Impulsive sources impart seismic energy into the subsurface in short duration events, wherein in each event substantially all the seismic energy is generated at the same time. Impulsive sources include, among others, air guns, water guns, dynamite and weight drop devices.
Another type of impulsive seismic energy source known in the art uses electromagnets to lift a moveable reaction mass towards a top block composed of laminated magnetic steel, which rests on top of a frame. The reaction of lifting the mass towards the top block is transferred via the frame to a base plate in contact with the ground. The movable mass is caused to move when the electromagnet therein is actuated to cause attraction between the electromagnet and the top block above it. A limitation to the foregoing type of mass lift impulsive seismic source known in the art is that the magnetic attractive force between the electromagnet and the top block is inversely related to the distance between them. Thus, on actuation, the attractive force between the electromagnet on the movable mass and the steel top block is smallest. Therefore, the maximum distance that the movable mass may be disposed from the top block when the movable mass is at rest is limited. Since no force can be transmitted to the ground once the attracting surfaces of the moving mass and top block meet, the above distance limitation will impose an absolute limit on the active stroke of the device, and thus the maximum attainable ground movement caused by the base plate. Further, because the attractive force between electromagnet and top block increases as the distance therebetween is reduced, the movable mass tends to accelerate during its travel toward the top block, limiting the accuracy of timing of seismic impulses generated by the source. The output of the mass-lift source known in the art also tends to have substantial energy above the frequency range that is detectable from the subsurface. It is desirable to have an impulsive seismic source that has improved timing control and has the capability to control the frequency content of the energy output thereof.